Pavement markers have become widely accepted as permanent installations for providing visible signals which mark traffic lanes and control the flow of traffic on roadways in combination with, or in place of, conventional painted traffic lines. A large number of such markers employ retroreflectors which retroreflect light emanating from oncoming vehicles to provide a signal visible to the operators of such oncoming vehicles.
Reflective pavement markers are constructed with an outer shell of material and are designed to withstand high impact forces expected to be encountered on the highway. One of the earlier types of markers of the style generally still used today is shown in the Heenan U.S. Pat. No. 3,332,327. In the basic structure shown in the '327 patent, the plastic retroreflector elements are first formed as part of the walls of a hollow shell, and then a layer of metal, by vacuum metallization, is deposited on the exposed faces of the cube corner retroreflector elements. Following that step, the "shell" is filled or "potted" with a rigid epoxy-type material. The resulting structure is relatively rigid and over the years has proven to be remarkably durable in use.
The '327 patent and others in the past have flat bottoms which generally are secured to the road surface by adhesives. This method of attaching the marker to the road has been adequate in most "sun belt" areas where snow plows are not used. The marker will stay attached to the normal pavement surface found in the United States even when the asphaltic material to which the marker is attached is heated by the mid-summer sun and will not move laterally, rotate or sink into the pavement as a result of contact with vehicle tires.
In "snow belt" areas, pavement markers are subject to the potentially greater dislocating force of a snow plow blade. To avoid this impact, the markers are held in a metallic casting which is buried into the road surface. This casting has rails on either side of the retroreflector to lift the blade relatively harmlessly over the marker housing. While such castings successfully protect the markers from contact with snow plow blades, they add a substantial expense to the cost of each marker and its installation.
As the use of road markers has spread outside of the Western Hemisphere, new road and impact conditions are challenging the previously accepted means for permanently securing markers to the pavement surface and protecting them from damage. For example, in Saudi Arabia, vehicles generally have a significantly greater axle load, higher (or no) speed limits and tires with higher air pressure. In addition, the quality of the asphaltic material in the pavement is well below that found in the United States and other Western countries and subject to patches of softness.
Use of the type of marker shown in the '327 patent for the road conditions of the type described above has resulted in sinking of the marker into the pavement due to downward impact forces on the soft, sun heated pavement. Moreover, attachment of the marker to the road by use of butyl adhesives rather than epoxy, as required in that country, results in an undesirable lateral sliding and rotational movement of the marker.
Prior markers that contained shafts which were inserted into the pavement were only partially successful at preventing undesirable movement. U.S. Pat. No. 3,516,337 shows a traffic button or road marker with a shaft having triangular shaped projections. More recently, Stimsonite Corporation, offered its Model 325 road marker which has a torque resistant stud to keep the marker in place. Neither of these markers could withstand the punishment of the extreme road and traffic conditions presented in places such as Saudi Arabia.
It is an object of the present invention to provide an improved, impact resistant, retroreflective pavement marker which resists downward, lateral and rotational movement upon impact, even when it is installed on soft or other poor quality roads.
It is another object of the present invention to provide an improved, impact resistant, retroreflective pavement marker which resists movement caused by impacts with high velocity, high speed vehicles having high tire pressures, even when installed on soft or other poor quality roads.
It is yet a further object of the present invention to provide an improved, permanently installed, impact resistant, retroreflective pavement marker which resists movement as a result of impacts with high velocity, high speed vehicles having high tire pressures, but which is relatively inexpensive to manufacture and install.
Other and further objects of the invention are apparent from the following discussion of the invention and the preferred embodiment.